1. Field of the Invention
The present invention relates to an antenna device and a radio communication apparatus that are capable of performing radio communication in multiple frequency ranges.
2. Description of the Related Art
The so-called “cellular system” uses frequency ranges of 800 MHz to 2 GHz for radio communication. In the cellular system, frequency ranges (hereinafter may be referred to as “bands”) to be used are specified for each country or region. Thus, the radio communication apparatus designed for use in various countries and regions has to be equipped with antenna devices that are capable of handling all bands specified for each country or region.
One example of such antennas that are capable of handling multiple bands is a two-branch antenna. The two-branch antenna is constituted by one RF (radio frequency) port and two elements for a low frequency range (hereinafter may be referred to as a “low band”) and a high frequency range (hereinafter may be referred to as a “high band”). An antenna device having the two-branch antenna achieves impedance matching for two bands, i.e., the low band and the high band, by using the two-branch antenna and an LC resonant circuit.
For example, Japanese Unexamined Patent Application Publication Nos. 2000-216716 and 2008-11329 describe technologies in which multiple antennas are assigned to respective multiple bands. That is, Japanese Unexamined Patent Application Publication Nos. 2000-216716 and 2008-11329 describe technologies about the so-called “antenna switching diversity” for which multiple antennae are provided and are used through switching. Thus, with the technologies, appropriate switching between the multiple antennas assigned to the respective bands allows the antenna device to handle all bands.
For example, Japanese Unexamined Patent Application Publication Nos. 2005-59121, 2007-235635, and 2007-143031 describe technologies for varying impedance by changing the antenna length or by using constants connected to multiple paths. As described in Japanese Unexamined Patent Application Publication Nos. 2005-59121, 2007-235635, and 2007-143031, when the antenna length is varied or impedance is varied using the constants connected to the paths, the antenna device can handle multiple bands by using a single antenna.